The present technology relates to a solid electrolyte cell and positive electrode active material. The present technology relates more specifically to a solid electrolyte cell including a solid electrolyte that contains no organic electrolyte solutions, and to a positive electrode active material used for the same.
Lithium-ion secondary cells utilizing doping and dedoping of lithium ions have been widely used in portable electronic apparatus and the like, because of their excellent energy density. Among such lithium-ion secondary cells, energetic research and development efforts have been under way on totally-solid lithium-ion secondary cells using, as an electrolyte, a solid electrolyte that contains no organic electrolyte solutions, from the viewpoint of safety and reliability.
As one form of the totally-solid lithium-ion secondary cells, thin-film lithium secondary cells have been under vigorous development. The thin-film lithium secondary cell is a secondary cell in which a current collector, active material and electrolyte, which make up the cell, are formed by thin films. Each thin film which makes up the thin-film lithium secondary cell is formed by using a film forming method such as sputtering and vapor deposition (e.g., see Non-Patent Document 1).
In a thin-film lithium secondary cell, an amorphous material such as LiPON which is obtained by subjecting Li3PO4 to substitution by nitrogen and LiBON which is obtained by subjecting LixB2O3 to substitution by nitrogen is used as a solid electrolyte. The ionic conductivity of these amorphous materials is about 10−6 S/cm which is significantly lower than that of typical liquid electrolytes of 10−2 S/cm. In the thin-film lithium secondary cell, the film thickness of the solid electrolyte is small (e.g., about 1 μm), and the distance traveled by Li is short. Therefore, the solid electrolyte made of the above amorphous material having a low ionic conductivity can show almost the same performance as liquid electrolytes.
On the other hand, in the thin-film lithium secondary cell, a positive electrode active material is one that limits the rate of the electrical conduction. It is typical to use, as this positive electrode active material, a lithium transition-metal oxide such as LiCoO2, LiMn2O4 and LiFePO4, as in a liquid-based lithium-ion secondary cell. Further, in addition to these, new lithium transition-metal oxides for use as the positive electrode active material have been proposed. For example, Patent Document 1 proposes a crystalline LiCu1+xPO4 as the lithium transition-metal oxide to be used as the positive electrode active material. These lithium transition-metal oxides (hereinafter referred to as “above-described lithium transition-metal oxides”) are materials which are low in ionic conductivity and electron conductivity.
In a thin-film lithium secondary cell, the thickness of the positive electrode active material layer is proportional to the cell capacity, so it is desirable to be as thick as possible in order to achieve a high capacity. However, in the thin-film lithium secondary cell, if the positive electrode active material layer made of a material low in ionic conductivity and electron conductivity is made thicker (e.g., 10 μm or more), it results in a very high internal impedance.
Therefore, it is difficult to commercialize a high-capacity thin-film lithium secondary cell having a thicker positive electrode active material layer, using the above-described lithium transition-metal oxides low in ionic conductivity and electron conductivity.
Besides, the above-described lithium transition-metal oxides are commonly used in a crystalline phase. Therefore, in a thin-film lithium secondary cell, in forming films of the above-described lithium transition-metal oxides, a crystalline phase is formed by such as heating of the substrate during the film formation, and post-annealing after the film formation.